1. Field of the Invention
The present invention relates to improved fluid filtration systems, particularly air filtration systems for supply of air to a work station or living quarters area where human or animal operatives are reliant upon air passing through the system, at least during times when toxic or harmful agents are present outside that area. The invention particularly provides a land vehicle, aircraft, water vessel or building ventilation system which is designed for operation in a toxic vapour or gas containing environment, such as might occur during armed conflict where chemical/biological warfare agents are deployed.
2. Discussion of Prior Art
It is known to remove contaminating agents from gases by use of low pressure activated carbon filter bed units, including those that have been impregnated with agents that increase sorption capacity for particular agents. These however are known to be limited in use when applied to systems for supplying air to an entire living area, as opposed to an individual, as they require frequent replacement due to limitations on service life, with resultant regimes of replacement at regular intervals needed if problems are to be avoided. Replacement filter canisters for individual masks take relatively little effort to fit, but filter replacement in a building or a vehicle provides a significant logistics problem. Furthermore, activated carbons are known to age even in the absence of active flow through them through action of moisture and heat, particulates cause damage and blockage, and exhaust fumes may poison the beds prior to intended use.
In order to avoid these problems other types of filter have been developed for removal of contaminants in air supplied in `collective protection` scenarios, particularly using pressure swing adsorption (PSA) technology. In GB 2238490 there is described a molecular sieve PSA unit for use in such applications whereby a full regenerative capability is provided by a compact unit allowing almost continuous energy efficient and reliable operation. This unit places an aerosol filter upstream and a particulate filter downstream of the molecular sieve beds. The document specifies that the PSA adsorber is included in lieu of an activated carbon or impregnated carbon filter. The use of a HEPA grade coalescing filter as upstream filter and a super-fine HEPA dust filter as downstream filter are recommended in order to achieve synergistic effect, with the result that the entire unit may be made compact.
This document further states that for added protection a high-pressure impregnated carbon filter may be placed downstream of the PSA unit, with a chemical agent vapour detector provided between the two to indicate when breakthrough had been achieved. However, this unit works on the basis of high pressure, and in the event that the compressor unit should fail, complete system failure will follow and the users will no longer be protected.
In the event of failure GB 2238490 prescribes `positive system shutdown` with NBC protection being switched to `an auxiliary system`. The nature of this auxiliary system is not suggested, but in any case is an otherwise redundant spare unit. The lack of back-up inbuilt into the main unit thus further requires a system for determining system failure, and initiation of the operation of the spare unit.